Silica is the generic term for minerals and other materials with the chemical formula SiO2. Silica collectively describes crystalline and non-crystalline forms. Crystalline silica (such as quartz, crystobalite, and tridymite) occurs in nature and can also be artificially produced by heating silicate glasses or other amorphous silicates.
Occupational exposure to crystalline silica dust constitutes a serious health hazard. This health hazard is also a concern for consumers using products containing crystalline silica. Silica is found in a large number of consumer products. Spackling patching and taping for drywall construction are formulated from minerals including crystalline silica, and silica flour is added to a multitude of consumer products such as toothpaste, scouring powders, wood fillers, soaps, paints and porcelain. Consumers may be exposed to respirable crystalline silica from such sources as abrasives, sand paper, detergent, cement and grouts. The primary health concerns in those exposed to silica dust are the fibrogenic capacity of the inhaled silica particles that can lead to the development of silicosis as well as an increased risk of tuberculosis. Nationally, the US Occupational Safety and Health Administration (OSHA) and the US National Institute for Occupational Safety and Health (NIOSH) set and regulate inhalation standards for silica dust. Internationally, the International Labour Organization (ILO) and the World Health Organization (WHO) have developed programs to reduce exposure of silica dust in developed and developing countries.
Workers in the foam glass manufacturing sector can be exposed to levels of crystalline silica during production. Consumers using foam glass blocks and powder for surface preparation by sanding, rubbing and/or scraping a surface to clean, abrade and polish such a surface may generate fine dust containing varying percentages of crystalline silica that may subsequently be inhaled. Workers in other industries can likewise be exposed to crystalline silica from foamed glass sources. The building material and insulation industries work with foamed glass in various forms and workers can be exposed in the cutting and handling of products made from foamed glass.
The manufacture of foamed glass includes a heating step that is conducive to transforming part of the amorphous ground glass (silica) into crystalline silica. The thermal profile required for production of foamed glass is often consistent with devitrification of the glass matrix. Crystalline silica, usually in the form of crystobalite, may be a devitrification product. In addition, some of the common foaming agents can accelerate the conversion rate of amorphous to crystalline silica and lower the temperature at which crystal growth occurs.
Crystalline silica is nucleated in vitreous, fused silica and siliceous glasses when the glass melt is cooled through the nucleation temperature range. Silica crystals grow in these glasses in a temperature range that is typically hotter than the nucleation range, although the two may overlap. The result is that during glass production, glass is cooled through the growth temperature range before it enters the nucleation range. Thus, siliceous glasses typically contain a substantial number of silica nuclei that have had little or no time to grow. However, when reheated for softening, such as inherent in the foaming process, these nuclei are thrust back into their growth temperature range and may now grow unchecked into silica crystals. Moreover, the reheating process takes the glass back through the nucleation range on its way to a softening temperature, where even more nuclei may be generated.
Thus, there is a need for a means for preventing or retarding further nucleation and growth of silica crystals in siliceous glass during the foaming process. The present novel technology addresses this need.